Abstract Many coral reef benthic algae exhibit morphological plasticity enabling them to persist in diverse habitats under different environmental pressures. Morphological plasticity has been previously observed in the brown alga Padina jamaicensis, which displays a foliose morphology under low grazing pressure and a turf morphology in habitats with medium to high herbivory levels. To determine whether a different species of Padina, P. boergesenii, exhibits morphological plasticity regulated by herbivory, we excluded macroherbivores from experimental plots containing P. boergesenii thalli on a rocky reef off Santa Marta in the Colombian Caribbean. The experiment used exclusion cages to test for effects of herbivores and was carried out in two contrasting oceanographic seasons: upwelling (low water temperature and high water transparency) and rainy (higher temperature and low water transparency). The morphology of P. boergesenii was significantly affected by herbivory but only during the upwelling season. ...